1. Field of the Invention
The present invention relates to an audio reproducing apparatus.
2. Description of the Related Art
Small loudspeakers are employed in minicomponent stereo sets and flat-screen television receivers. In such applications, the enclosure of the loudspeaker (speaker box) is accordingly small. The resonance frequency f0 of the loudspeaker is as high as or higher than 100 Hz.
An audio signal having the resonance frequency f0 or lower may be supplied to the loudspeaker. With the frequency being lowered, the fundamental component is increasingly lowered while distorted components (harmonic components) sharply increases in the sound output pressure thereof.
Audio apparatuses having small loudspeakers cannot sufficiently reproduce low-frequency component lower than the resonance frequency f0 of the loudspeaker.
Two methods may be contemplated in the reproducing of the audio signal:
(1) an equalizer is used to boost the low-frequency component, and
(2) a harmonic overtone component of the low-frequency component is output to achieve a low-frequency sound effect.
The method (1) provides the low-frequency sound effect by reinforcing the frequency component in the resonance frequency f0 band of the loudspeaker.
The method (2) takes advantage of the hearing of humans. More specifically, the sound of each musical instrument is composed of a fundamental component and harmonic components, and the ratio of the fundamental component to the harmonic components determines the tone of the sound. It has been psychoacoustically proved that if a sound without the fundamental component but with the harmonic components thereof is output humans hear as if the fundamental component is also output. The method (2) is based on such a human hearing property.
FIG. 14 illustrates an audio apparatus. A loudspeaker 5 is used to improve the low frequency sound effect. An audio signal S1 is supplied to a high-pass filter 2 at an input terminal 1. As shown in FIG. 15A, a middle to high-frequency component equal to or higher than the resonance frequency f0 of the loudspeaker 5 is extracted and supplied to an adder 3. The audio signal S1 at the terminal 1 is supplied to a band-pass filter 7. As shown in FIG. 15B, a low-frequency component S7 falling within a frequency band from f0/2 to f0 is extracted and supplied to a pitch shifter 8.
The pitch shifter 8 doubles the frequency of the supplied low-frequency component S7. As shown in FIG. 15C, a frequency multiplied component S8 within a band from f0 to 2f0, namely, the low-frequency component S8, is output.
The low-frequency component S8 is supplied to the adder 3 to be added to the middle to high-frequency component S2. The adder 3 outputs an audio signal S3 with the low-frequency component S8 as the low-frequency component S7 reinforced as shown in FIG. 15D. The audio signal S3 is output to the loudspeaker 5 via a power amplifier 4. The loudspeaker 5 thus emits an acoustic sound having frequency characteristics of FIG. 15D, namely, an acoustic sound with the low-frequency component S8 as the reinforced low-frequency component S7.
The sound of the low-frequency component S7 is not output from the loudspeaker 5 and corresponds to the fundamental component. The corresponding low-frequency component S8 is output from the loudspeaker 5. A listener hears as if the low-frequency component S7 is actually output. Even with the small loudspeaker 5, the low frequency sound effect is thus provided.
It is generally said that humans suffer from no unpleasant hearing impression on the low-frequency component S8 lower than 200 Hz even if the frequency multiplied component S5 is generated by multiplying the low-frequency component S7.    Japanese Unexamined Patent Application Publication No. 8-213862 discloses one such technique.